Computer assisted tomography (CAT Scan) is a method of medical diagnosis which utilizes x-rays, generated by firing an electron beam at an anode in a region of high vacuum. In this application it is necessary to generate a high density of x-rays for a relatively short period of time, rather than a lower density of x-rays for a longer period of time, as the former is better tolerated by humans. For this purpose, a high-power electron beam is advantageously utilized to bombard an anode, thus producing x-rays, but the process can result in heat and radiation-induced degradation of the metal anode. Thus, the anode is commonly mounted on a shaft which rotates extremely rapidly (6,000 to 12,000 revolutions per minute) so that a fresh anode surface is continually presented to the electron beam. As the anode surface rotates out of the beam, it is allowed to cool before being re-introduced into the beam, and anode degradation is avoided. Also, the x-ray-generating anode and supporting apparatus is generally violently accelerated back and forth across the top of the CAT Scan apparatus such that analysis from various required angles relative to the human body is carried out in a relatively short period of time. Low weight is thus desirable, and elimination of the necessity of continuous pumping to maintain high vacuum is advantageous. Anode-supporting shaft speeds of from 6,000 to 12,000 revolutions per minute (RPM) are ideally attainable for extended periods in this application, and mechanical stability of the shaft arrangement is thus extremely important.
Charge builds in the anode and in the shaft as the electron gun fires upon the anode, thus the shaft must be efficiently electrically grounded or arcing across shaft-support bearings will occur, ruining them over time. Conventional grounding inside a vacuum environment via, for example, mercury slip rings, carbon brushes, or gold foil brushes will result in particulate matter contaminating the vacuum, and at very high shaft rotation speeds (6,000-12,000 RPM) efficient grounding directly at the shaft becomes impossible in any region. In addition, conventional grounding near bearings can cause particulate matter to ruin the bearings.
Ferrofluidic seals have been utilized to provide a hermetic seal against gas and other contaminants in applications similar to that described above. Ferrofluidic seals have been utilized to prevent contaminants from reaching the disk area in computer magnetic disk storage units, for robotic actuators designed for use in ultra-pure vacuum processing of semiconductor wafers, and for pumps in refineries and chemical plants.
Ferrofluidic seals are normally installed to remain stationary about rotating shafts, but can be installed to seal a stationary shaft about which a hub rotates. The seals operate generally through employment of ferrofluid in a gap between the rotating shaft and stationary seal surfaces, and include an annular magnet for providing a desired magnetic flux path which retains and concentrates the ferrofluid in a seal-tight liquid O-ring in the gap. Ferrofluidic seals typically include a permanent annular ring magnet polarized axially, and a pair of magnetically-permeable annular pole pieces which sandwich the magnet, so that inner peripheral edges of the pole pieces extend toward and form a close, non-contacting gap with the exterior rotating shaft surface.
A single-stage ferrofluidic seal is created by placing a single annular pole piece in close proximity with and surrounding a shaft and in magnetic communication with a single magnet. Ferrofluid is retained in the pole piece/shaft gap by the magnetic field created by the magnet, which field follows a magnetic circuit containing the magnet, pole piece, the gap and the shaft. A second annular pole piece also in close proximity with the shaft and in magnetic communication with the other pole of the magnet can advantageously be employed in a single-stage seal. The gap between this second pole piece and the shaft generally contains no ferrofluid but enhances the magnetic flux across the gap within which ferrofluid is retained in the single-stage seal, thereby increasing the pressure capacity of the seal.
The ferrofluid seal can comprise more than one stage, that is, the seal may comprise a series of discrete pole pieces, or a pole piece (or pole pieces) may include a plurality of ridges and grooves, each ridge in close proximity with the shaft and defining an annular gap between the pole piece and the shaft. Ferrofluid is retained in several or all of these gaps to form a multi-stage ferrofluidic seal.
In another embodiment, a series of ridges and grooves is formed in the shaft, rather than in the pole piece or pole pieces, to define a multi-stage ferrofluidic seal in close proximity with an annular pole piece/magnet arrangement surrounding and in close proximity with the shaft.
The single pole piece, the series of pole pieces, the plurality of ridges in a pole piece (or pole pieces), or the ridge(s) in a shaft need not each have the same concentric radius, that is, the size of the resultant annular gap need not be equal from pole piece to pole piece or from ridge to ridge. In addition, the pole pieces and/or ridges need not be of the same width, nor of the same geometric design. Some of the pole pieces or ridges may be tapered to form an annular wedge-shaped gap having a sloping wall extending radially toward the axis of the shaft.
Variation of the width and/or taper of the pole pieces can result in selective ferrofluid retention in one or more annular gaps to create, selectively, one or more ferrofluid seals where originally each annular gap defined a seal. In addition, a gap created by a tapered pole piece will normally retain ferrofluid for a longer period of time, and tapered pole pieces or shafts prevent ferrofluid splashing at high shaft or hub rotation speeds. U.S. Pat. No. 4,357,021, issued Nov. 2, 1982, and U.S. Pat. No. 4,890,850, issued Jan. 2, 1992 and assigned to the same assignee of the present invention, describe the above-noted gap-selective retention of ferrofluid.
Conventional ferrofluid seals employed to seal high-vacuum systems, either under static or dynamic conditions, typically permit periodic bursts of air to pass the seal and to be introduced into the vacuum system. The periodicities of the bursts of air depend on the seal design and operating conditions. Furthermore, when the ferrofluid seal is employed for the first time after being in a static condition, a burst of air is typically introduced into the vacuum system. In a seal arrangement in which such bursting occurs, the high vacuum environment must be addressed by continuous pumping means, which adds weight to the overall apparatus.
A non-bursting, multiple-stage ferrofluid seal, designed to eliminate this problem, is described in U.S. Pat. No. 4,407,518, issued Oct. 4, 1983. In this invention, a first multi-stage annular seal is arranged in series with a second, single-stage annular seal along a shaft, the second seal being on the high-vacuum side of the first seal. The first multi-stage annular seal is designed and constructed to withstand the entire pressure drop between atmosphere (or high pressure) and high vacuum. An annular region between the first and second seals is maintained at a pressure between atmosphere (or high-pressure) and high vacuum, generally at a slight pressure, or a vacuum slightly poorer than the high vacuum. Thus, any bursting of the first, multi-stage seal resulting from the pressure differential between the atmosphere or high-pressure region and the intermediate region affects only the intermediate region, which is addressed by rough vacuum and is relatively undisturbed. The vacuum on either side of the second, single-stage seal is substantially identical, or at most, the pressure differential across the second seal is less than the pressure required to cause the second seal to burst. Thus, no bursting of the second seal occurs. The result is an improved, longer-lasting vacuum in the high-vacuum region.
However, the bearings supporting the shaft in the above-noted invention are arranged, for mechanical stability, in series with and surrounding the first and second seals and the intermediate vacuum region. Thus, at least one bearing is exposed to the high-vacuum region in this invention. This arrangement results in bearing outgassing into the high-vacuum region, contaminating and spoiling the high-vacuum region more quickly, and requiring continuous pumping.
One approach for isolating the high-vacuum region from bearing outgassing involves a "cantilevered" arrangement in which a multi-stage seal is arranged about a shaft with all shaft-supporting bearings in series with the seal but on the atmosphere or high-pressure side thereof. (Commercially available from Ferrofluidics Corporation, Inc., of Nashua, N.H.) For this arrangement, however, the bearings are spaced relatively close together rather than relatively at the ends of the shaft, and a shaft which is mechanically unstable at high rotation speeds results.
Additionally, the ferrofluid seal arrangements described above, when utilized to seal a shaft on which an anode or other electrical-charge-generating device is mounted, are subject to arcing across the seals when the shaft is not efficiently electrically grounded. Such arcing causes ferrofluid to "gel" and ruins the seals, and as noted above, grounding of a high speed rotary shaft is difficult.
Accordingly, a general purpose of the present invention is to provide a differentially-pumped, multiple-stage, mechanically-stable, non-bursting ferrofluid seal apparatus for sealing a rotary shaft which passes between an environment at a first pressure, which is generally atmospheric pressure or high pressure, and an environment at a second pressure, which is less than or equal to the first pressure and is generally a high vacuum. The apparatus is designed to be mechanically stable at very high shaft rotation speeds yet arranged such that bearing outgassing and ferrofluid seal bursting into the high-vacuum region is eliminated. The apparatus is further designed such that no continual mechanical vacuum pumping is required, and the shaft is electrically grounded remotely from high-speed rotation and remotely from vacuum and bearings. The above objects adapt the apparatus for the purpose of supporting a high-speed rotary anode, which anode is exposed to an electron beam for x-ray generation in a CAT Scan apparatus or the like.